1. Field of the Invention
This invention relates to a driving circuit for an alternating current drive type of capacitive flat matrix display panel, that is a driving circuit for a thin film EL (electroluminescent) display device.
2. Description of the Prior Art
As an example, a double insulation type or triple layer, structure thin film EL device is composed as follows.
As shown in FIG. 5, transparent electrodes 2 which are made from In.sub.2 O.sub.3, and which are formed in a band shape are provided on a glass substrate 1 in parallel to each other. On these transparent electrodes 2, a dielectric material 3 such as Y.sub.2 O.sub.3, Si.sub.3 N.sub.4, Al.sub.2 O.sub.3, an EL layer 4 made of ZnS in which an activator such as Mn is doped, and a dielectric material 3' such as Y.sub.2 O.sub.3, Si.sub.3 N.sub.4, TiO.sub.2 or Al.sub.2 O.sub.3 are respectively formed in thin films of 500 to 10000 A thickness successively applied by a thin film technology such as vapor deposition or sputtering to form three layers. Thereafter band-shaped back plates 5 made of Al.sub.2 O.sub.3 are formed in parallel over the three layers in the direction perpendicular to that of the transparent electrodes 2.
Since the above thin film EL device is formed in such a manner that the EL material 4 sandwiched in between the dielectric materials 3 and 3' is interposed between the electrodes, it can be considered as a capacitive device from the view point of circuit equivalency. As can be clearly seen from FIG. 6 in which voltage-brightness characteristics are shown, the thin film EL device is driven by a relatively high level voltage substantially equal to 200 V. The thin film EL device has the capacity to emit bright light due to its a.c. electric field and exhibits a long life.
In order to decrease power consumption in at the modulation drive system of a display device in which a thin film EL device of the type described above is used, the applicant of the present invention has previously proposed a driving device comprising a scanning side driver IC as a driving circuit for scanning side electrodes. The scanning side driver IC comprises transistors which apply negative voltage to data side electrodes and transistors which apply positive voltage to the same. On the other hand, in order to serve as a driving circuit for the data side electrodes, the driving device comprises a data side driver IC which has transistors for charging the EL layer up to the modulation voltage, transistors for discharging the EL layer, and diodes each connected in the inverse direction to the direction of electric current flow of the corresponding transistors. With the structure described above, modulation drive may be performed on the data side with the use of the charging and discharging transistors driven by display data. On the other hand, on the scanning side, field reverse drive is performed with the use of N-ch transistors and P-ch transistors. Furthermore, successive drive of scanning lines may be performed with the polarities of a writing waveform applied to picture elements reversed every other scanning line. As a result of this application of symmetrical write pulses with opposed polarities, a reliable driving device is obtained that exhibits the capacity to horizontally scan one line within a short time, and to apply alternating pulses of good symmetry to the EL layer (see, for example, Unexamined Japanese Patent Publication No. SHO 61-282895) corresponding to Ser. No. 864,509.
Since the above mentioned driving device comprises, as shown in FIG. 4(a), a charge side transistor UT of the data side driver IC which is made of a bipolar type of NPN transistor, no electric current is conducted from a common line Vcc to the charge side transistor UT when the charge side transistor UT is switched off. However, as shown in FIG. 4(b), current flow will occur only in a case where the data side electrode is negative.
The reason why this current flow occurs is that although the base potential of the charge side transistor UT is zero in the data side driver IC to make this transistor nomenclature, a parasitic diode disposed between the base and the emitter is biased in the forward direction when the potential of the data side electrode is negative because the transistor is of the NPN type. As a result of this, the base current flows to cause the transistor UT to be turned on, thereby allowing a collector current to flow. The data side electrode inevitably becomes negative because the thin film EL display device needs to be driven in an alternating current manner. Therefore, if the thin film EL display device is driven by a conventional driver IC, an excessive amount of current will be lost.
Furthermore, in conventional drive circuits, the electrical charge which has been accumulated in the thin film EL display device is fully consumed by resistance factors within the driving circuit at the time of discharge. As described above, since an active type (self-luminescent type) of display basically consumes a large amount of electricity, it is desired to decrease the electricity consumption.